Bandwidth Calculator

time = (size × 8) ÷ speed

size

Amount of data to move, as a byte count (GB)

speed

Link speed as a bit rate (Mbit/s)

time

How long the transfer takes (seconds)

The 8 converts the byte count into bits, because the size and the speed are quoted in different currencies. Once the two sides are both bit rates it is a plain division. Rearranging gives the other two answers: multiply the speed by the time to get how much data fits, or divide the data by the time to get the speed a transfer actually ran at. The reference table below applies the first form to a range of common line speeds, so you can read a transfer time off it without entering anything.

1. A 5 GB game update on a 100 Mbit/s line

   1. Known: size = 5 GB, speed = 100 Mbit/s.
   2. Turn the size into bits: 5 GB = 5 × 1000 MB = 5000 MB = 5000 × 8 = 40 000 megabits.
   3. Divide by the speed: 40 000 ÷ 100 = 400 seconds.
   4. That is 6 minutes 40 seconds.
   5. Check by the other route: 5 GB ÷ 12.5 MB/s = 400 s, where 12.5 MB/s is 100 Mbit/s ÷ 8. ✓

   The route through megabytes per second is the one people do in their heads, and it is the same arithmetic — 12.5 MB/s is what a 100 Mbit/s line delivers once the 8 is accounted for.

2. 1 GB on a 1 Gbit/s fibre line

   1. Known: size = 1 GB, speed = 1000 Mbit/s.
   2. 1 GB = 1000 MB = 8000 megabits.
   3. 8000 ÷ 1000 = 8 seconds.
   4. Check: 1000 Mbit/s is 125 MB/s, and 1000 MB ÷ 125 MB/s = 8 s. ✓

   Ten times the speed is a tenth of the time, which is the point of quoting both ends of this scale — the same file is a coffee break on copper and a blink on fibre.

3. How much data a 100 Mbit/s line moves in an hour

   1. Known: speed = 100 Mbit/s, time = 3600 seconds (1 hour).
   2. Data = speed × time = 100 × 3600 = 360 000 megabits.
   3. Turn bits into bytes: 360 000 ÷ 8 = 45 000 megabytes = 45 GB.
   4. This is the question a data cap asks: a 50 GB monthly allowance is used up in a little over an hour at this rate. ✓

   Note that this is the line rate, not what a real download achieves — see the limitations below for the overhead a measured transfer adds.

4. A 500 MB file on the same 100 Mbit/s line

   1. Known: size = 0.5 GB (500 MB), speed = 100 Mbit/s.
   2. A tenth of the data takes a tenth of the time of the 5 GB case: 400 ÷ 10 = 40 seconds.
   3. Check directly: 0.5 × 8000 ÷ 100 = 40 seconds. ✓

   Enter 0.5 in the size box with the unit set to GB, or switch the unit to MB and enter 500 — the calculator converts to the base unit before doing anything.

This is the time at the line rate, not a prediction of a real download. Ethernet, IP and TCP each add header bytes, so a measured transfer moves less payload than the raw bit rate suggests, and a transfer over a congested link or Wi-Fi shares the air with everything else in range. A rule of thumb is that a well-behaved transfer achieves 85–95% of the link rate on a wired connection, and considerably less over Wi-Fi. The page also ignores the far end: a server that uploads at 10 Mbit/s caps the transfer no matter how fast your download is, and a slow disk or a slow CPU can matter for very fast links. Files sizes are decimal (1 GB = 1000 MB) unless you pick MiB or GiB, where the powers of two apply.

How do I work out how long a download will take?

Divide the amount of data by the transfer speed, once both are in the same currency. A 5 GB file is 40 000 megabits, so at 100 Mbit/s it takes 400 seconds — 6 minutes 40 seconds. The shortcut people use is to turn the speed into megabytes per second first: 100 Mbit/s ÷ 8 = 12.5 MB/s, and 5000 MB ÷ 12.5 MB/s = 400 s.

Why is the download speed 8 times smaller than the advertised speed?

Because the two are quoted in different units, not because anything is being lost. Providers sell megabits per second and download managers report megabytes per second, and there are 8 bits in a byte. A 100 Mbit/s line is the same thing as a 12.5 MB/s line. If your speed test reads in MB/s, multiply by 8 before comparing it with the figure on your bill.

Is the answer the same as my actual transfer time?

It is the time at the line rate, so a real transfer is usually a little slower. Every packet carries header bytes, the far end may throttle you, and Wi-Fi shares the channel with other devices. On a wired connection a healthy transfer manages 85–95% of the link rate. To measure yours, run the calculation backwards: enter the file size and the time it actually took, and the calculator returns the speed you really got.

How much data can I move in a month?

Set the time box to the number of seconds and leave the size box empty. One hour on a 100 Mbit/s line is 45 GB, so a 50 GB data cap lasts about 66 minutes of sustained full-rate transfer. Streaming at 5 Mbit/s uses the same math the other way: about 2.25 GB per hour, or 1.6 TB if left running all month.

What is the difference between GB and GiB?

GB is a decimal unit, 1000 MB or one billion bytes, and it is what drive makers and ISPs use. GiB is a binary unit, 1024 MiB or 2 to the 30th power bytes, and it is what most operating systems display. The two differ by about 7%, which is why a 1 TB drive shows up as 931 GiB in a file manager. Both are available in the size unit menu, so pick whichever matches the number you are starting from.

Does the calculator handle uploads and streams?

Yes — a bandwidth figure is a bandwidth figure in either direction, and the page does not care what the data is. For an upload, use the upload speed your provider quotes, which is often much lower than the download speed on domestic connections. For a stream, remember that the figure you want is the bitrate of the stream itself rather than the capacity of the line, and that a stream only buffers ahead by a few seconds before playback starts.

• Prefixes for binary multiples — NIST
• The International System of Units (SI Brochure) — BIPM